An internal combustion engine such as a diesel engine for a vehicle emits exhaust gas from an engine body. Exhaust gas includes particulate matters such as soot and soluble organic fraction (SOF). In general, a particulate filter is provided to an exhaust passage of an engine for collecting particulate matters, so that particulate matters are restricted from being discharged to the atmosphere.
When particulate matters such as soot are collected using a filter, the particulate matters accumulate on the filter, and the filter may be clogged. When the filter is clogged, pressure of exhaust gas may increase on the upstream of the filter. As a result, engine performance and fuel efficiency of the engine may be degraded. Therefore, in general, temperature of exhaust gas and a filter is increased to a specific temperature, which is equivalent to combustion temperature of soot, so that soot accumulating on the filter is burned and removed in a filter recovery operation. Thereby, the filter is capable of being restricted from causing clogging due to accumulation of soot.
However, when the filter recovery operation is performed in a condition, in which a large amount of soot accumulates on the filter, the accumulating soot quickly burns all together, and temperature of the filter may excessively increase. On the contrary, when the filter recovery operation is performed in a condition, in which a small amount of soot accumulates on the filter, the fuel efficiency of the engine may be degraded due to excessively repeating the filter recovery operation. Therefore, the amount of soot accumulating on the filter needs to be correctly detected for properly performing the filter recovery operation.
When a catalyst such as platinum is supported in the filter, soot accumulating in the vicinity of the catalyst on the filter is capable of being oxidized and burned by oxidation caused by the catalyst, even when temperature of the filter is less than the combustion temperature of soot.
A filter supporting a catalyst is disclosed in EP0766993B1 (JP-A-09-94434). In this filter structure, pieces of catalyst are supported on the surfaces of partition walls of the filter, and furthermore, pieces of catalyst are supported on the surfaces of pores (small holes) formed in the partition walls of the filter. Therefore, when soot accumulates around the catalyst supported on the surfaces of the partition walls and the pores, the soot is capable of being oxidized in the filter, even when temperature of the filter is less than the combustion temperature of soot. Thus, soot can be oxidized and burned at low temperature using the filter supporting the catalyst.
However, soot may not be oxidized and removed by the catalyst uniformly throughout the filter. For example, when temperature of exhaust gas increases, temperature difference may arise throughout the filter. In this case, soot accumulating on the filter may be partially oxidized by the catalyst.
In this situation, even when the amount of soot accumulating on the filter is the same, differential pressure between the upstream and the downstream of the filter may not be the same. Specifically, the differential pressure of the filter may vary depending on both the amount of soot, which is oxidized by the catalyst, and a region, in which oxidized soot accumulates on the filter. Accordingly, it is difficult to estimate the amount of soot, which accumulates on the filter, in accordance with the differential pressure of the filter.
An accumulating amount estimating device is disclosed in JP-A-2003-166413. In this accumulating amount estimating device, when the differential pressure of the filter does not reflect the amount (soot accumulating amount) of soot accumulating on the filter, the device estimates the soot accumulating amount in accordance with a past trend of the differential pressure of the filter in a predetermined period. Specifically, when a variation in differential pressure of the filter is less than a predetermined degree in the predetermined period, the device estimates the soot accumulating amount on the basis of the past variation in the differential pressure of the filter. Thereby, the accumulating amount of soot is capable of being estimated, even when the accumulating amount of soot is hard to be estimated in accordance with the difference pressure of the filter.
However, in the accumulating amount estimating device disclosed in JP-A-2003-166413, the accumulating amount of soot is estimated on the basis of the past trend of the differential pressure of the filter in a condition, in which the differential pressure of the filter does not represent the accumulating amount of soot. Accordingly, the estimation of the accumulating amount is not accurate in this device.